Continuous steel casting method

ABSTRACT

Oxide inclusions in continuously cast steel are constrained to be well dispersed and predominantly of very small size by controlling the environment and nature of the tundish-to-mold stream continuously to exclude and remove oxygen and gasses containing oxygen so as to inhibit oxide formation except as is incidental to the cooling and freezing of the metal in the mold itself.

United States, Patent [191 Kenney 1 Oct. 8, 1974 CONTINUOUS STEEL CASTING METHOD [76] Inventor: Malachi P. Kenney, Shepherd Hill Rd., East Aurora, NY. 14052 [22] Filed: July 18, 1968 [21] Appl. No.: 745,728

[52] U.S. Cl 164/66, 75/96, 164/82 [51] Int. Cl... B22d 21/02, B22d 23/00, B22d 11/00 [58] Field of Search 164/66, 57, 275, 273, 281,

[56] References Cited UNITED STATES PATENTS 2,889,596 6/1959 Savage et a1 164/283 2,983,973 5/1961 Parlanti 164/66 X 2,997,384 8/1961 Feichtinger 164/66 X 3,094,424 6/1963 Ratcliffe 164/281 UX 3,281,903 11/1966 Ross 164/66 3,392,009 7/1968 Holmes et al.... 164/66 X 3,402,757 9/1968 Halliday 164/66 3,459,537 8/1969 Hornak 75/49 3,465,811 9/1969 De Castelet 164/281 3,467,167 9/1969 Mahin 164/66 X FOREIGN PATENTS OR APPLICATIONS 411,245 4/1966 Switzerland 164/66 1,395,648 3/1965 France 164/66 Primary Examiner-J. Spencer Overholser Assistant ExaminerVernon K. Rising Attorney, Agent, or FirmBean & Bean 1571 ABSTRACT Oxide inclusions in continuously cast steel are constrained to be well dispersed and predominantly of very small size by controlling the environment and nature of the tundish-to-mold stream continuously to exclude and remove oxygen and gasses containing oxygen so as to inhibit oxide formation except as is incidental to the cooling and freezing of the metal in the mold itself.

1 Claim, 4 Drawing Figures PAIENTEuum 81974 MALACHI P,KENNEYY Em BUM ATTORNEYS 1 v i l CONTINUOUS STEEL CASTING METHOD A BACKGROUND OF THE INVENTION It is well recognized that inability to control the size of nonmetallic (oxide) inclusions incidental to the production of steel by continuous casting methods has prohibited the continuous caster from supplying steel for high quality application such as for bearings, aircraft parts or the like.

Although the formation of these inclusions is known to be due to the presence of oxygen in the molten steel, careful control of oxygen content, such as is successful in producing high quality reduced ingot steel, is not ad equate in producing high quality continuously cast steel. The reason for this is that although as compared with ingot cast steels, the size of nonmetallic inclusions in continuously cast steel actually may be smaller initially, owing to the fact that ingots are of relatively large cross section while continuous cast billets are of relatively small cross section, the former undergo a much greater reduction in cross section during subsequent processing. Consequently, the nonmetallic inclusions in an ingot product are elongated, broken up and greatly reduced in cross section so that ultimately they may be much smaller in size than in a comparable product made from a continuously cast billet. For example, a typical ingot having approximately 600 square inches of cross section when reduced to a 1 inch square bar must undergo about 40 times the reduction required to reduce a typical 4 inch square continuously cast billet to the same size.

Thus, the attainment of low oxygen content in continuously cast steel, which compares with that leading to high quality products from reduced ingot steel, is unsuccessful in producing what are ordinarily considered to be high quality continuously cast steel products. Unfortunately, the obvious solution of still further reducing the oxygen content is not one which at present appears to be practicably attainable. That is to say, an ideal situation with regard to either continuous cast or reduced ingot cast steels would be to reduce the oxygen content to zero, but this ideal is at present, far from attainable. r

The methods used to control the oxygen content of continuously cast steel include the usual chemical additions to the furnace and the ladle (i.e., the addition of silicon and/or aluminum to the molten steel) and the use of means to prevent steel-air contact, particularly in the teeming stream from ladle to tundish and in the stream between tundish and mold. The theory underlying these proposals is that prevention of steel-air contact will exclude oxygen pick-up from the atmosphere and allow the ordinary chemical methods of deoxidation to reduce the oxygen content of the steel to such a low value as minimizes the presence of nonmetallic inclusions.

Although some improvement is obtained by these methods, the results are far from satisfactory inasmuch as the presence of substantial quantity of large sized nonmetallic inclusions remains a problem.

BRIEF SUMMARY OF THE INVENTION This invention is directed to method and apparatus for continuously casting steel, in which method the formation of oxides due to the inevitable presence of oxygen in solution is controlled so that the oxides predominately are present in the cast steel in the form of widely dispersed inclusions of small size.

The process involves the use of a tundish into which the molten steel is teemed from a suitable ladle, whereafter the molten steel is directed in one or more streams to a conventional continuous casting mold or molds. Chemical deoxidation of the steel is effected by ladle addition and the oxides formed by such chemical deoxidation in the tundish, as well as oxides carried over from the ladle largely are separated out in the form of slag floating on the surface of the molten steel in the tundish.

The tundish-to-mold stream is protected by a gaseous envelope to: I

a. prevent the introduction of exogenous oxygen,

b. remove oxygen and oxygen compounds from the molten steel stream, and

c. inhibit the formation of oxides in the tundish-tomold stream and limit the formation of oxides substantially to that which is incidental to the freezing of the metal in the mold.

It has been found that if the above effects are carried out, substantially all of the oxide inclusions in the continuously cast steel will be less than about 20 microns in size and well dispersed whereby the quality of the steel materially is improved.

Specifically, these effects are accomplished by enclosing the tundish-to-mold stream within a flexible, porous sleeve, into which high purity argon gas is introduced under pressure sufficient to cause a continuous flow of the inert gas which constantly flushes the inert atmosphere and thereby removes oxygen, carbon monoxide and gaseous compounds which may contain oxygen from the inert atmosphere, whereby to stimulate and continuously maintain the atmosphere in condition to favor the diffusion of oxygen and gaseous compounds containing oxygen into such atmosphere.

Although the exact mechanism by which'the present invention operates is not as yet known, it is believed that the mechanism involves diffusion of oxygen in solution in the molten steel into the inert atmosphere, reduction of oxides already formed, and the formation of carbon monoxide which diffuses into the inert atmosphere; and that these and possibly other gaseous diffusions distinctly inhibit oxide formation except as is incidental to the cooling and freezing of the metal in the mold. The cooling and freezing of the metal in the mold takes place rather rapidly and this is believed to lead almost exclusively to the formation of oxides which are well dispersed and of the small size heretofore mentioned.

l have found that large size, quality degrading nonmetallic inclusions in continuously cast steel may be largely eliminated not so much by a consideration of the oxygen content of the steel itself, although this is of importance, but mainly by continuously subjecting the tundish-to-mold stream or streams to an atmosphere conducive not only to the exclusion of oxygen contact withthe steel, but also to removal of oxygen or oxygencontaining gas from the steel at this point.

Preferably, I effect this deoxidation and oxygen exclusion of the tundish-to-mold stream by enclosing the stream in a high purity argon atmosphere while continuously changing the atmosphere so that at all times the argon possesses substantially zero partial pressure of oxygen and oxygen-containing gas. Under these conditions, the oxides which do form in the solidifying steel,

due to precipitation of oxygen therefrom as the steel cools and freezes, are present as widely dispersed nonmetallic inclusions which are so small as do not materially affect the quality of the steel.

It is important to note that whereas it is of interest to maintain the oxygen content of the steel to a value below about 100 PPM in the present process, this value lies within normal ranges for continuously cast steel. To illustrate, conventionally cast continuous steel exhibits an oxygen content within the range of about 60 to about 160 PPM and do contain substantial quantity of large size, quality-degrading nonmetallic inclusions whereas with the present process the oxygen content of the continuously cast steel ranges only slightly lower (between about 40 to about 90 PPM) but with the substantial absence of large size nonmetallic inclusions, the oxides instead being present predominantly as well dispersed inclusions of sizes less than about 20 microns.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING FIG. 1 is a vertical section taken through the apparatus for practicing the continuous casting process according to the present invention;

FIG. 2 is an enlarged section taken through the shroud which encloses the tundish-to-mold stream;

FIG. 3 is a section on reduced scale taken along the plane of section line 3-3 in FIG. 2 and showing the cutoff valve device; and

FIG. 4 is an enlarged transverse section taken along the plane of section line 44 in FIG. 3.

BRIEF DESCRIPTION OF THE INVENTION With regard to FIG. 1, the reference character indicates in general a ladle, only the lower portion of which is shown in FIG. 1 and which will be understood to contain molten steel 12 obtained by furnace processing. The ladle is provided with a suitable nozzle or orifice 14 in its bottom and a conventional stopper rod (not shown) is utilized in conjunction therewith to control the flow of steel from the ladle 10 to the underlying tundish 16. The tundish is provided with a pair of nozzle or orifice portions 18 and 20 for delivering the molten steel to the reciprocatory open ended molds 22 and 24. It will be understood that the molds 22 and 24 are of entirely conventional construction as used in the continuous casting industry and are provided with the usual mechanism (not shown) for effecting vertical reciprocatory motion of these molds continuously to follow and then strip from the billets 26 and 28 formed.

As can be seen more clearly in FIG. 2, the bottom wall 30 of the tundish is provided under each nozzle 18 and 20 with a valve plate assembly indicated generally by the reference character 32 and the upper faces 34 of the molds 22 and 24 form seats for the collars 36. The lower face 37 of each valve plate assembly 32 forms a seat for the upper collar 38, the two collars 36 and 38 being provided with flanges 40 and 41 which form seats for and are urged apart by the compression spring 42 and interposed between the collars and their respective seating surfaces 34 and 37 are packing or sealing rings 43 and 44 received in suitable grooves in the collars and adapted, under the action of the spring 42, to effect seals at these points.

Enclosed within each spring 42 is a porous sleeve 45 having its opposite ends clamped upon the collars 38 and 36 by suitable bands 46 and 47, it being understood that the sleeves 45 are of sufficient length to accommodate for the maximum extension between the molds 22 and 24 and the tundish 16.

The valve plate assembly 32, as may be seen more clearly in FIGS. 3 and 4 includes a main body portion 50 provided in its upper surface with a slot 52 receiving the cutoff plate member 54. The main body portion 50 of the valve may be of circular profile presenting a relatively large central opening 58 in register with the tundish nozzle 18 whereas the plate 54 is adapted to be moved between covering and uncovering relation to the tundish nozzle 18. As may be seen in FIG. 4, the plate 54 is provided with a graphite insert 60 which bears against the bottom surface of the tundish, the insert being provided to prevent welding of the plate 54 to the tundish when the plate is moved into covering relation to the tundish nozzle.

Each of the collars 36 is provided with inert gas inlet means which, as can be seen in FIG. 2, may comprise a tube 62 suitably anchored as by the nuts 64 and terminating in a cross tube portion 66 which is closed at its opposite ends 68 and 70 and provided in its lateral side faces with openings 72 to admit the inert gas into the enclosure 74 formed by the porous sleeve 45. Each sleeve 45 is formed of suitable material such as a woven asbestos material having sufficient refractory properties to withstand the heat of its environment and yet having suitable porosity to allow leakage of the inert gas therethrough. The sleeve 45 is of course flexible to accommodate for the vertical reciprocatory motion of the molds 22 and 24.

The inert gas, preferably high purity argon, is introduced into the enclosures 74 under sufficient pressure to assure a leakage or bleed-off rate of the inert gas from the enclosures 74 which is effective to change and flush the inert gas atmospheres in the enclosures 74 so as to maintain the atmosphere substantially free of oxygen or oxygen-containing gas whereby to favor deoxidation of the tundish-to-mold stream 76. Preferably, the bleed rate of the inert gas is in the order of 23.44 cubic feet per minute while casting the two, four inch square, billets 26 and 28 (in the order of 48 cubic feet per ton of steel) and typically an internal pressure within each enclosure 74 of about one and one-half pounds per square inch gauge is sufficient to assure such leakage rates. A suitable pressure regulating valve and pressure gauge, not shown, are provided to allow the operator to control the bleed rate.

The tundish construction is of entirely conventional nature except for the provision of a pair of dams and 82 upstanding from the bottom walls and extending between the opposite side walls of the tundish. These dams are placed so as to trap the ladle-to-tundish stream 84 therebetween and cause the incoming molten metal to flow upwardly toward the surface 86 of the tundish reservoir before passing downwardly to the nozzles 18 and 20. The surface 86 is of course maintained at a level higher than the dams and means may be provided, not shown, automatically to control the ladle-to-tundish flow for maintaining a fixed metallo static head in the tundish.

The dams 80 and 82 are provided to enhance the natural separating effect of the tundish by aiding nonmetallic inclusions in the molten steel in finding their way to the surface 86 whereat they may remain as slag, thereby preventing carry-over into the molds.

Calcium and/or silicon are used in the ladle as a deoxidizing agent and the agent preferably is used in the amount of about 5 pounds per ton of steel, sufficient to establish an oxygen content in the tundish of about 60-190 PPM. Thedams 80 and 82 help in assuring that the oxides formed by the aforementioned deoxidizing agents find their way to and remain as slag on the surface 86 of the tundish reservoir so that slag carry-over is also minimized. Typically, the depth of the tundish reservoir may be about 17 inches using two 11/16 inch diameter nozzles 18 and 20 to form 4 inch square billets 26 and 28. The tundish-to-mold streams 76 are about inches in length and the casting time for a 22 ton heat is approximately 45 minutes.

With the above conditions prevailing, using high pu rity argon gas supplied from a suitable source at a pressure of about 1.5 PSIG and at a through-rate of about 48 cubic feet per ton of steel, the steelwill be characterized by the substantial absence of nonmetallic inclusions larger than about microns and may be produced on a quantity and repetitive basis. The nonmetallic inclusions which are present in the steel are mainly in the form of extremely fine and widely dispersed oxides such as manganese, silicon, aluminum and calcium oxides. v

Five test heats, each of approximately 22 tons capacity, were cast of A181 C4050 steel, one strand or billet of which was subjected to protection and deoxidation of its tundish-to-mold stream according to this invention while, the stream forming the other strand was exposed to air. A comparison of the results illustrating the percent reduction between the control strands and the strands cast according to this invention is as follows:

Percent Reduction Oxygen analysis (average) NMl count (average) 74% NM! size (average) 78% From the above, it becomes clear that the factor of oxygen content reduction in the billet is not the only factor involved inasmuch as the reduction in oxide or nonmetallic inclusion count and size is far in excess of the reduction in oxygen content. Moreover, as noted earlier, the oxygen content of the steel obtained by the present invention overlaps the range obtained by conventional processing, the former as noted being about 40-90 PPM with the latter being about 60-160 PPM.

Rather, the presence substantially only of oxide inclusions in the present invention which are extremely fine (less than about 20 microns) and widely dispersed appears to be due at least in large part to the combined effect of protection and deoxidation of the tundish-tomold stream. Tests conducted with a protective atmosphere around the tundish-to-mold stream but without provision for deoxidation at this point, although showing reduction in nonmetallic inclusions as compared with an unprotected stream, do not compare favorably with the results produced by this invention. A protective atmosphere in the absence of the deoxidizing effect may be expected to produce only about one-sixth to about one-third the effect produced by this invention insofar as quantity of large size nonmetallic inclusions is concerned. I

From the above, it will be clear that this invention in-' volves simultaneously the protection and deoxidation of the tundish-to-mold stream by enclosing the same in an inert atmosphere and changing the atmosphere at a rate continuously to effect deoxidation of the stream. As has been mentioned, the continuous changing of the inert atmosphere is believed to so favor the diffusion of oxygen and oxygen containing gas from the stream to the atmosphere as inhibits the formation of oxides in the steel except as is incidental to the cooling and freezing of the steel in the mold, whereat they may form as well dispersed and very small inclusions in the steel. In particular, it appears that any inert atmosphere having substantially zero pressure of oxygen and carbon monoxide will, if changed at a rate to retain such substantially zero partial pressures, so favors the deoxidation of the steel as to accomplish the desired result. Accordingly, other and different inert atmospheres, rather than argon as specifically disclosed, may be used.

What is claimed is:

1. The method of continuously casting a high quality steel characterized by minimization of nonmetallic inclusions of a size greater than about 20 microns, which comprises the steps of:

a. making a heat of molten steel and pouring it into a ladle; I

b. chemically deoxidizing the molten steel in the ladle to establish an oxygen content in the molten steel which is in the range of about 60-190 ppm;-

c. teeming the molten steel from the ladle into a tundish while simultaneously streaming the molten steel from the bottom of the tundish into a continuous casting mold and controlling the flow of molten steel in the tundish to direct the molten steel coming into the tundish back toward the surface of the molten steel before passing to the continuous casting mold;

d. passing the tundish-to-mold stream through a flexible, porous sleeve which is sealed at opposite ends to the tundish and the mold whereby to form a protective enclosure around said tundish-to-mold stream;

e. introducing a high purity inert gas into said protective enclosure under sufficient pressure to cause a continuous flow of such gas into such enclosure and then outwardly thereof through said porous sleeve; and

f. continuously maintaining the flow of gas of step (e) during casting at a rate sufficient continuously not only to exclude and to remove oxygen and gasses containing oxygen from said enclosure but also to deoxidize the tundish-to-mold stream and thereby constrain nonmetallic inclusions formed in the solidified steel in the mold to be well dispersed and of sizes less than about 20 microns while reducing the oxygen content of step (b) and obtain an oxygen content in the solidified steel which is within the range of about 40 to about parts per million. 

1. The method of continuously casting a high quality steel characterized by minimization of nonmetallic inclusions of a size greater than about 20 microns, which comprises the steps of: a. making a heat of molten steel and pouring it into a ladle; b. chemically deoxidizing the molten steel in the ladle to establish an oxygen content in the molten steel which is in the range of about 60-190 ppm; c. teeming the molten steel from the ladle into a tundish while simultaneously streaming the molten steel from the bottom of the tundish into a continuous casting mold and controlling the flow of molten steel in the tundish to direct the molten steel coming into the tundish back toward the surface of the molten steel before passing to the continuous casting mold; d. passing the tundish-to-mold stream through a flexible, porous sleeve which is sealed at opposite ends to the tundish and the mold wherEby to form a protective enclosure around said tundish-to-mold stream; e. introducing a high purity inert gas into said protective enclosure under sufficient pressure to cause a continuous flow of such gas into such enclosure and then outwardly thereof through said porous sleeve; and f. continuously maintaining the flow of gas of step (e) during casting at a rate sufficient continuously not only to exclude and to remove oxygen and gasses containing oxygen from said enclosure but also to deoxidize the tundish-to-mold stream and thereby constrain nonmetallic inclusions formed in the solidified steel in the mold to be well dispersed and of sizes less than about 20 microns while reducing the oxygen content of step (b) and obtain an oxygen content in the solidified steel which is within the range of about 40 to about 90 parts per million. 